XenAPI: Support Nova services independently from hypervisor

The ability to run Nova compute services independently from the XenServer host which is running the virtual machines would enable:
* Easier development model where the XenServer hypervisor could be running in a virtual machine (e.g. VirtualBox), but with Nova services running on the main development box
* Consolidation of Nova compute services in a production environment, having a specialised Nova compute environment which supports multiple hypervisors (still with a 1:1 relationship between Nova Compute and Hypervisor)

This blueprint makes the necessary changes to enable this independent compute mode of deployment

It looks like this is all merged now. There were 0 unit tests for the last change in the series, which troubles me given none of the cores that approved it asked for them. There should definitely be some unit tests added for this after the fact. -- mriedem 20160629